In a sign of U.S. confidence that the weeklong assault on Libya has tamed Moammar Gadhafi’s air defenses, the Pentagon has reduced the amount of naval firepower arrayed against him, officials said Sunday.

The move, not yet publicly announced, reinforces the White House message of a diminishing U.S. role — a central point in President Barack Obama’s national address Monday night on Libya. The White House booked Defense Secretary Robert Gates and Secretary of State Hillary Rodham Clinton on three Sunday news shows to promote the administration’s case ahead of the speech.

Yet Gates, asked whether the military operation might be over by year’s end, said, “I don’t think anybody knows the answer to that.”

At least one of the five Navy ships and submarines that have launched dozens of Tomahawk cruise missiles at Libyan targets from positions in the Mediterranean Sea has left the area, three defense officials said. They spoke on condition of anonymity in order to discuss sensitive military movements.